The present invention relates to a control circuit for automatically controlling the feed rate of parts.
Prior art control circuits have been devised for automatically controlling the speed of conveyors or other motion systems in accordance with the frequency of parts passing a given point. Invariably, however, these prior art systems are accompanied by the caveat that they are unsuitable for controlling inductive loads. This is primarily due to the fact that a conventional control system will cause an inductive motor to overheat after a short period of operation. The overheating problem arises when controlling inductive loads due to the continuous cycling of the motor that is inherent with automatic motor speed control. While many conventional motors can withstand such operation for a longer period of time without seriously overheating, inductive motors will overheat, in some instances, in a matter of a few minutes. Even many conventional motors cannot be continuously operated under the control of prior art control systems over extended periods of time without encountering overheating problems.
Accordingly, it is desirable to devise a system for automatically controlling the feed rate of parts that can be used not only with motion systems driven by conventional motors, but also with systems employing inductive loads. Thus, it is the primary objective of the present invention to provide a control system that not only eliminates the overheating problem with the motive means of the system, but actually causes the motive means to operate cooler than it would operate under normal conditions.
In addition, with conventional prior art control systems, the speed of the motor often becomes erratic and the torque of the motor is substantially diminished when operated at no load or at low-speed control settings. Accordingly, there exists with conventional speed control systems certain speed control settings below which a motor will not effectively operate. Thus, for those applications wherein it is desirable to operate at relative slow speeds without a significant loss in torque, conventional speed control systems are not practical.
Accordingly, it is an additional objective of the present invention to provide a motor control system that permits the motor to be operated at low speed control settings without causing erratic operation of the motor and a corresponding loss in torque.
Another drawback of conventional speed control systems is the lack of means for controlling the spacing between parts in addition to controlling the overall feed rate of the parts. The additional control capability can be of significant importance in certain testing applications, for example, wherein it is desirable to maximize the quantity of parts being tested and at the same time maintain the accuracy of the testing procedure by insuring that adjacent parts do not pass the testing apparatus so closely as to cause erroneous test results. The present invention avoids this problem by providing circuitry that controls the reaction time of the load to the passage of parts by the sensing device. In this manner, the gap between parts can be accurately controlled to the extent that the motor can actually be made to rapidly slow down and then rapidly speed up following the detection by the sensing device of each individual part.
Generally speaking, the present invention comprises a light source and a photo-sensing device disposed on either side of a passageway so that parts moving along the passageway will interrupt the light beam between the light source and the photo sensing device. The photo-sensor is adapted to provide an output pulse whenever the light beam from the light source is interrupted. The output pulses from the photo-sensor are provided to a monostable multivibrator which converts the signal to a sequence of fixed amplitude fixed width pulses. The sequences of pulses from the multivibrator are then integrated and provided to the base of a transistor which controls the level of charge on a storage capacitor.
The storage capacitor is charged by a supply voltage until a level of charge is attained that exceeds the breakover potential of a threshold device. When this occurs, an electronic switching device is fired which, in turn, controls the operation of the motor.
Accordingly, it will become apparent that the greater the number of parts passing the sensing device, the lower the average level of charge on the storage capacitor, and the slower the average speed of the motor. Similarly, the fewer the number of parts passing the sensing device, the greater the average charge level on the capacitor, and the faster the average speed of the motor.
Also included, is a high wattage, low valued resistor which, is connected in series with the motor to absorb the heating spikes which result from the cyclic operation of the motor inherent with proportional control. In addition, a separate discharge path is provided from the capacitor to ground through a normally back-biased diode, to rapidly return the charge level on the capacitor to its quiescent state after each cycle of the supply voltage. In this manner, the problem commonly referred to as "skip-cycling" is avoided, which permits the motor to be operated at slow speed control settings without erratic motor operation or significant loss in torque.